1. Field of the Invention
The present invention relates in general to control systems for internal combustion engines, and more particularly to control systems of a type which controls a secondary air fed to the exhaust system of the internal combustion engine.
2. Description of the Prior Art
In automotive internal combustion engines, there is a type in which a secondary air is fed to an exhaust system to burn and thus reduce harmful CO and HC in an exhaust gas from the engine. Usually, the secondary air is fed to the exhaust passage upstream of a catalytic converter, so that the exhaust gas fed to the converter can have a sufficiently high temperature due to burning of CO and HC. With this arrangement, remaining CO and HC in the exhaust gas is effectively burnt in the converter, and thus, the harmful CO and HC in the exhaust gas can be greatly reduced.
However, feeding the catalytic converter with the secondary air sometimes induces an excessive heating of catalyst of the converter, which tends to lower the catalytic activity and shorten the life of the catalyst. In view of this, as is described in Japanese Patent Second Provisional Publication 53-9663, usually, the secondary air feeding is made only at the time when, like at engine idling, CO and HC tend to increase. In fact, in order to obtain a stable running of the engine, a somewhat richer air/fuel mixture is fed to the engine during idling, which however in turn causes an increase of CO and HC in the exhaust gas.
However, even when feeding of the secondary air to the exhaust system is made only at the above-mentioned limited time, excessive air feeding to the system sometimes induces a certain temperature drop in the catalytic converter, which lowers the activity of the catalyst. In fact, the concentration (viz., lightoff performance) of fresh air in the exhaust gas, which allows sufficient exhaust gas oxidation at the lowermost allowable temperature, has been determined. For keeping the dilution of the exhaust gas with the secondary air at a given level, it is preferable to control the amount of the secondary air in proportion to that of the exhaust gas from the engine.
However, in the conventional system disclosed by the above-mentioned publication, the secondary air feeding fails to satisfy the above-mentioned proportional supply. That is, in the system, the secondary air feeding is powered by an air pump which is directly driven by the engine. That is, the amount of the secondary air is controlled so as to be proportional to the rotation speed of the engine, not to the amount of the exhaust gas from the engine.
In view of the above, electrically controlled air pumps have been proposed for controlling the secondary air fed to the exhaust system of the engine. Most of them are of a type in which an electric control valve disposed in a secondary air feeding pipe is controlled to vary its opening. However, in this type, the air pump is subjected to a wasteful operation due to marked pressure drop of the secondary air produced when the air passes through the control valve. Furthermore, since the pressure ratio between the pressure appearing upstream of the control valve (viz., the discharge pressure of the pump) and the pressure appearing downstream of the control valve varies, satisfactory proportional air control is not obtained.